1. Field of the Invention
The present invention relates to a zoom lens and an optical apparatus having the zoom lens, and more particularly, though not exclusively, a zoom lens that can be used in an optical apparatus such as a silver-halide film camera, a digital still camera, a video camera, a telescope, a binocular, a liquid crystal projector, and a copying machine.
2. Description of the Related Art
Recently, image sensors having a large number of pixels are used in image pickup apparatuses such as digital cameras.
With respect to photographic lenses that are used in such image pickup apparatuses to form an image, the market has desired zoom lenses having a high zoom ratio.
In addition, with respect to photographic lenses used in such image pickup apparatuses, the market has desired zoom lenses in which chromatic aberration, which affects a degree of color blurring and a degree of resolution in a white light source condition, is corrected, and in which spherical aberration and coma, which are related to monochromatic imaging performance, are corrected, as well.
With regard to a method of reducing chromatic aberration, U.S. Pat. No. 5,132,848 and U.S. Pat. No. 6,594,087 each discuss an optical system that uses an extraordinary partial dispersion material as an optical material. In addition, U.S. Pat. No. 5,731,907 and U.S. Pat. No. 5,638,215 each discuss an achromatic optical system that uses a liquid crystal material having relatively high dispersion and extraordinary partial dispersion characteristics.
In order to implement a zoom lens that has a high performance, a high field angle, and a high zoom ratio, it is useful to optimally set a lens configuration.
Generally, when a refractive power of each lens unit in a zoom lens is increased, the amount of movement of each lens unit during zooming can be reduced. Accordingly, the entire length of the zoom lens can be readily shortened.
However, if the refractive power of each lens unit is simply increased, the amount of aberration variation occurring during zooming becomes large. In particular, as a zoom lens has a higher zoom ratio, chromatic aberration of magnification varies in a larger amount.
For example, in a negative lead type optical system, chromatic aberration of magnification with respect to g-line light occurs in an over-corrected direction at the wide-angle end and in an under-corrected direction at the telephoto end. As a result, color blurring occurs in a peripheral portion of an image plane, thus deteriorating an image quality.
In a method of reducing chromatic aberration with an extraordinary partial dispersion material such as a fluorite arranged in an optical path, unless the extraordinary partial dispersion material is provided with an appropriate refractive power and arranged at an appropriate position, chromatic aberration can increase at either one of the wide-angle end and the telephoto end, while chromatic aberration can be corrected at the other.
In addition, the material discussed in U.S. Pat. No. 5,731,907 and U.S. Pat. No. 5,638,215 is a liquid material. Accordingly, the characteristics such as refractive index and dispersion characteristics considerably vary due to variation in temperature. Thus, such a material does not have useful environment resistance.